Development of an Intelligent Control Strategy for Minimization of Very Fast Transient Over Voltage in GIS Switchgear

Abstract

Gas insulated substations (GIS) have been used in power systems over the last three newlinedecades because of their high reliability, easy maintenance, small ground space newlinerequirement etc. In India also, a few GIS units are under various stages of newlineinstallation. The GIS is totally enclosed and therefore is free from any atmospheric newlinecontamination. However the life of GIS is affected by several factors such as: newlineconductive particles, partial discharges and contamination (decomposition products, newlinemoisture, etc.). Conductive particles inside the enclosure are known to reduce the newlinebreakdown level of Gas insulated systems. Although GIS has been in operation for newlineseveral years, a lot of problems encountered in practice need further understanding. newlineSome of the problems associated are: Switching operations generate very fast newlinetransient over voltages (VFTOs), VFTOs may cause secondary breakdowns inside a newlineGIS and transient enclosure voltages (TEV) outside the GIS. Prolonged arcing may newlineproduce corrosive / toxic by products and support spacers can be weak points when newlinearc by products and metallic particles are present. For these reasons, VFTOS generated in a GIS should be considered as an important newlinefactor in the insulation design. In a GIS, Very Fast Transient Overvoltages (VFTOS) newlineare caused by two ways: a. Due to switching operations and b. Line to enclosure newlinefaults. The switching operations may be disconnector switch, circuit breaker or earth newlineswitch. Since, the contact speed of disconnector switches are low, restriking occurs newlinemany times before the interruption. Each restrike generates very fast transient over newlinevoltages with different levels of magnitude. Further, disconnector switches are used newlineprimarily to isolate the operating sections of an HV installation from each other as a newlinesafety measure. Beyond this, they must also be able to perform certain switching newlineduties, such as load transferred from one bus bar to another or disconnection of bus newlinesections, circuit breakers etc. Contd.... newline